Compact PTC resistor

ABSTRACT

A compact resistor device comprises a body of ceramic material of positive temperature coefficient of resistivity having a large number of passages extending through the body between opposite ends of the body, thereby forming very thin webs of the resistor material between adjacent passages. Coatings are formed on the resistor material along the inner walls of the passages to serve as ohmic contacts. The coatings in alternate passages are connected together at one end of the resistor device and the coatings in the other passages are connected together at the opposite end of the device to serve as device terminals. When the device terminals are connected to a power source, current flows through very thin webs of resistor material between ohmic contacts in adjacent body passages. The resistor device is particularly useful in current limiting applications requiring low, room-temperature resistance and in heat-exchanger applications where a fluid to be heated is directed through the passages.

This is a division of application Ser. No. 721,727, filed Sept. 9, 1976,now U.S. Pat. No. 4,107,515.

Resistors formed of ceramic materials of positive temperaturecoefficient of resistivity (PTC) are used in many applications ascurrent limiting devices and as self-regulating heaters. When electricalcurrent is directed through such materials, the materials tend to heatand to display increasing resistivity so that current flow in theresistor is reduced and so that the rate of heat generation by theresistor is decreased. When the rate of heat generation reachesequilibrium with the rate of heat dissipation from the resistor, theresistor temperature stabilizes and limits the resistor current to aselected level. The initial, room temperature resistivity of a PTCmaterial and the rate of change of resistivity with temperature arecharacteristic of the material and the materials used in such resistorsare commonly chosen to display a sharp, anomalous increase inresistivity at a particular temperature, thereby to stabilize heating ofthe resistor at about that temperature while also reducing resistorcurrent to a very low level at the stabilizing temperature.

For some current limiting resistor applications, it would be desirableto provide a bistable current limiting resistor which could carry asubstantial current load under normal conditions without self-heating toany significant extend but which would be rapidly self-heated tosignificantly limit resistor current if an over-current condition shouldoccur. Such a bistable current limiting device could be arranged inseries with an electric motor to carry normal motor operating currentswith only moderate variations of resistor temperature but could berapidly self-heated in response to overcurrent conditions in the motorif the motor were overloaded, thereby to restrict motor current to asafe level before overheating of the motor winding could occur. However,it has been difficult to obtain resistor materials having desiredpositive temperature coefficients of resistivity which also display thelow room-temperature resistivities necessary to permit a compact, ruggedresistor to carry normal motor currents without immediately heating theresistor materials to their high resistivity temperatures.

For some uses of PTC resistors as self-regulating heaters, it has alsobeen suggested that the resistor body be provided with a large number ofpassages extending between opposite ends of the body. When ohmiccontacts are then formed at each end of the body, and when electricalcurrent is directed through the body while fluid flows through the bodypassages, the fluid is efficiently heated without risk of resistoroverheating even if the fluid flow should be blocked or interrupted.However, such fluid heating resistors have been subject to undesirableresistance banding. That is, thermal gradients occurring in the resistormaterial between opposite ends of the resistor as fluid is passedthrough the resistor passages permit narrow bands of high resistivitymaterial to be established across the direction of current flow. As aresult, current is reduced resulting in reduced heater efficiency and,in addition, such narrow bands of the resistor material may be subjectedto local electrical fields which are excessively high and which tend todegrade the PTC materials.

It is an object of this invention to provide a novel and improvedresistor device of a material of positive temperature coefficient ofresistivity; to provide such a resistor device which is particularlyadapted for use as a self-regulating fluid heater; to provide such aresistor device which is particularly adapted for use as a currentlimiting device having very low room-temperature resistance; and toprovide such a resistor device which is of rugged, compact, reliable andinexpensive structure.

Briefly described, the novel and improved resistor device of thisinvention comprises a resistor body preferably formed of a ceramicmaterial of positive temperature coefficient of resistivity. The body isprovided with a large number of small, closely spaced passages whichextend in side-by-side relation to each other between two opposite endsurfaces of the resistor body. Ohmic contacts are formed on the resistormaterial extending along the inner walls of the body passages, typicallyby coating the inner walls of the passages with a very thin, highlyadherent metal coating. The ohmic contact layers in alternate ones ofthe body passages are then connected together at one of the end surfacesof the resistor device while the ohmic contact layers in the other bodypassages are connected together at the opposite end surface of thedevice. In this way, each group of commonly connected contact layersserves as one terminal of the resistor device. Accordingly, whenelectrical current is directed between the device terminals, the currentflow is between the ohmic contact layers in pairs of adjacent bodypassages so that the current passes through only the very thin webs ofthe resistor material which separate the adjacent body passages fromeach other.

In this arrangement, the thickness of the resistor material between theohmic contact coatings is very small so that the resistor devicedisplays very low room-temperature resistance even though ceramicmaterials having conventional room-temperature resistivities are used inthe device. On the other hand, because the resistor body passages aresmall and closely spaced, the resistor device accomodates very largeeffective ohmic contact areas even though the resistor device is of acompact, rugged structure, Accordingly, the resistor device is adaptedto carry a relatively large current load while maintaining a low currentdensity in the thin web portions of the resistor body. With thisstructure, the resistor device of this invention is adapted to beconnected in series with an electrical motor to carry normal motorcurrents without self-heating the resistor material to its highresistivity state but to rapidly heat the material to its highresistivity state to limit motor current to a safe level if the motorshould be overloaded.

The described resistor structure also has advantages when used as aself-regulating heater. For example, if a fluid to be heated is directedthrough the passages of the resistor device of this invention, thermalgradients occurring in the body do not tend to subject the resistormaterials to excessive electrical fields at any locations within theresistor body. That is, although the resistor body may tend to be morerapidly heated near one end of the device as fluid flows in the devicepassages, the higher resistivity occurring at that location does notrestrict flow of electrical current between ohmic contact areas at otherlocations within the resistor body. Thus, heating efficiency is notexcessively reduced. Further, because the webs of resistor materialbetween the ohmic contact coatings in adjacent body passages are verythin, thermal gradients which may occur across the thickness of the thinwebs are not significant.

Other objects, advantages and details of the novel and improved resistordevice of this invention, of methods for making the resistor device, andof systems using the resistor device appear in the following detaileddescription of preferred embodiments of the invention, the detaileddescription referring to the drawings in which:

FIG. 1 is a perspective view of the novel and improved resistor deviceof this invention;

FIG. 2 is a partial section view to enlarged scale along line 2--2 ofFIG. 1;

FIG. 3 is a schematic view illustrating operation of the resistor deviceof FIG. 1;

FIG. 4 is a schematic view illustrating application of the resistordevice of FIG. 1 to a solid state motor protection system;

FIG. 5 is a section view similar to FIG. 2 illustrating alternateembodiment of this invention particularly adapted for use in a currentlimiting application;

FIG. 6 is a block diagram illustrating steps in manufacture of theresistor device of FIG. 1; and

FIG. 7 is a section view similar to FIG. 2 illustrating a step in analternate process for making the resistor device of this invention.

Referring to the drawings, 10 in FIGS. 1 and 2 indicates the novel andimproved resistor device of this invention which is shown to include aresistor body 12 formed of a material having a positive temperaturecoefficient of resistivity. Preferably the resistor body is formed of aconventional ceramic resistor material such as lanthanum-doped bariumtitanate or the like and preferably the resistor material is selected todisplay a sharp, anomalous increase in resistivity when the resistorbody is heated to a particular temperature.

The resistor body 12 has a plurality of passages 14 extending in apattern in side-by-side relation to each other between two opposite endsurfaces 16 and 18 of the resistor body. For example, the passages arearranged in a pattern with at least three rows of passages each havingmore than three passages per row and typically the pattern of passagesincludes a large number of passages on the order of 49 to 64 passages.Preferably each passage has a square or rectangular cross-section or thelike as shown so that each passage is separated from adjacent passagesin the body by thin webs 20 of resistor material of equal and generallyuniform thickness.

In accordance with this invention, the inner walls of each resistorpassage are covered with an adherent, electrically conductive coating 22by which ohmic or other suitable contact is made to the resistormaterial. The coatings or contact elements in alternate body passagesare then electrically connected together at one end of the resistor bodywhile the coatings formed on the walls of the other body passages areelectrically connected together, preferably at the opposite end of theresistor body. Preferably for example, as shown in FIG. 1, the coatingsof ohmic contact material 22 in alternate body passages are electricallyconnected together by a pattern 24 of electrically conductive materialcoated on the end surface 16 of the resistor body, the pattern havingportions 24.1 extending between the ends of pairs of said other bodypassages to interconnect the ends of the coatings formed in thealternate body passages. A corresponding pattern 26 of electricallyconductive material is coated on the opposite end surface 18 of theresistor body (see FIG. 2) for electrically interconnecting the coatings22 of ohmic contact material in the other body passages. The materialused in forming the ohmic contact coatings 22 can also be used informing the interconnection patterns 24 but the interconnection patternmaterial need not be in ohmic contact with the resistor material. Forclarity of illustration, the interconnection patterns 24 and 26 areshown to extend completely around ends of the coatings 22 to which therespective patterns are connected but it will be understood that thepatterns need connect to the coatings in the passages at only a singlelocation.

In this arrangement, the interconnection patterns 24 and 26 serve asterminals of the resistor device 10. Accordingly, when the terminals areconnected to a power source as is schematically illustrated in FIG. 3,current flows through the thin webs 20 of the resistor body between theohmic contact coatings 22 connected to the pattern 24 and the ohmiccontact coatings 22 connected to the pattern 26 as indicated by thearrows 28 in FIG. 3. Where the webs 20 of the resistor body are verythin, the room temperature resistance of the resistor body 10 is verylow even though conventional ceramic PTC materials having conventionalroom temperature resistivities are used in the resistor body. On theother hand, where the interconnection patterns 24 and 26 are eachconnected to a plurality of ohmic contact coatings 22, the effectiveohmic contact area of each device terminal is relatively large.Accordingly, current density in the webs 20 of the resistor body is verylow even though the resistor device is adapted to carry a substantialcurrent load.

In addition, because the webs 20 of the resistor body are typically verythin, temperature gradients which may occur across the thickness of thewebs are not significant. That is, although the center portion of a web20 may have a temperature T_(c) as indicated in FIG. 3 while the outersurface portion of the web has a lower temperature indicated at T_(o) inFIG. 3, the thin nature of the web keeps this gradient from beingsignificant. On the other hand, if a fluid to be heated is directedthrough the device passages as indicated by the arrows 30 in FIG. 3 sothat the device temperatures T_(c) ' and T_(o) ' are relatively higherthan the temperatures T_(c) and T_(o), the higher resistivities of theresistor material where the temperatures T_(c) ' and T_(o) ' occur donot in any way restrict current flow between ohmic contact areas atother locations within the resistor body. Accordingly, the occurrence ofthe noted temperature gradient along the length of the device does notresult in any of the resistor material being subjected to an excessivelyhigh, local, electrical field.

For example, where the resistor device 10 is to be used as a bistablecurrent limiting device requiring a low room temperature resistance lessthan about 0.3 ohms but where a conventional ceramic resistor materialhaving a room-temperature resistivity of about 36 ohm-centimeters isused in the resistor body 12, the ratio of ohmic contact area providedby each of the device terminals 24 and 26 to the thickness of theresistor material through which current is directed has to be greaterthan about 120 centimeters. Accordingly, the device 10 is typicallyformed of a conventional lanthanum-doped titanate material having anempirical formula of Ba₀.968 Pb₀.030 La₀.002 Ti O₃. Such a resistormaterial has a room temperature resistivity of about 36 ohm-centimetersand a Curie temperature of about 140° C. and would display a sharp,anomalous increase in resistivity of about 10⁵ ohm-centimeters when theresistor material is heated above its anomaly temperature to 200° C. Theresistor body is provided in the form of a square prism 0.6 centimeterson a side of 2.0 cm long having 16 passages of a square cross-section0.1 centimeters on a side arranged in 4 rows of 4 passages each withwebs 20 between the passages having a thickness of about 0.04centimeters. The inner walls of the passages 14 are then coated withmolecular bonding aluminum ink, with an electroless nickel plating, orwith another electrically conductive coating material 22 conventionallyused in forming ohmic contents on such ceramic resistor materials. Theelectrically conductive interconnection patterns 24 and 26 are thenapplied to opposite end faces of the resistor body to form deviceterminals as above decribed.

In this arrangement, the effective ohmic contact surface area of each ofthe device terminals is about 4.8 square centimeters providing a ratioA/L of ohmic contact area (A) to web thickness (L) of about 120centimeters. Thus, the resistor device is provided with aroom-temperature resistance of 0.3 ohms and is adapted to carry a normalcurrent load of 1.0 ampere in a 12 volt system with a current density ofonly about 200 milliamperes per square centimeter in the webs 20 of theresistor body. The structure of the resistor device permits the deviceto easily dissipate the heat generated in the device to an air ambientat 25° C. while carrying the current load without increasing resistortemperature more than about 12° C. However, if resistor current isabruptly increased to above 3.0 amperes, the resistor device is rapidlyheated to its anomaly temperature for increasing device resistance to 50ohms for typically restricting resistor current to 0.2 amperes.

Accordingly, the resistor device 10 is adapted for use as a bistablecurrent limiting device to provide solid state protection for anelectrical motor to prevent overheating of the motor windings as isschematically illustrated in FIG. 4. That is, where a conventionalelectrical motor having a main winding 32 and a start winding 34 areconnected as shown in FIG. 4 using a conventional motor startingresistor 36 of a PTC material for effectively cutting out the startwinding 34 after motor starting has occurred, the terminals 24 and 26 ofthe resistor device 10 are connected as shown for interposing theresistor device 10 in series between the motor and a power sourceindicated by the line terminals 36. Where the resistor device 10 isprovided with characteristics as above described to match the operatingcharacteristics of the selected motor, the resistor device is adapted tocarry the normal operating current of the motor including the initiallyhigher currents occurring as normal variations in motor operation takeplace but is adapted to rapidly heat to the anomaly temperature of theresistor materials in the device 10 on the occurrence of a locked rotorcondition or the like in the motor for restricting motor current to asafe level to prevent excessive overheating of the motor winding 32. Forexample, the resistor device 10 exemplified above is adapted for usewith an electrical motor having normal operating currents varyingbetween 1.0 and 2.0 amperes but is adapted to restrict motor current to0.2 amperes within 20 seconds after the occurrence of a motor overloadcurrent of 5.0 amperes.

The resistor device 10 is manufactured in a variety of ways inaccordance with this invention, but basically includes the process stepsillustrated in FIG. 6. In this regard, the formation of themulti-passages body 12 indicated at 40 in FIG. 6 is accomplished in themanner illustrated in U.S. Pat. No. 3,790,654, for example, or in otherwell known ways as may be preferred. The outer side and end surfaces ofthe resistor body are then covered with a masking material in anyconventional way, as by dip or brush coating or the like, while leavingthe ends of the body passages open as indicated at 42 in FIG. 6. Theinner walls of the body passages are then coated with the ohmic contactmaterials 22 in any conventional way as by dipping the masked resistorbody in a suitable coating bath such as a bath of molten metal, orelectroless nickel plating baths or the like as indicated at 44 in FIG.6. The marking materials are then removed as indicated at 46 in FIG. 6.Preferably the coating deposited on the passage walls are subjected to aheat treatment for facilitating making of ohmic contact to the resistormaterials. As various methods for coating ceramic resistor materialswith ohmic contact materials are well known, the process for coating theinner passage walls is not further described but it will be understoodthat any conventional techniques are used.

After formation of the coatings 22, the coatings 22 are interconnectedin a variety of ways within the scope of this invention as indicated at48 in FIG. 6. For example, as shown in FIG. 5, conductive members arepressed into the ends of appropriate passages 14 to engage the ohmiccontacts 22 inside the passages. The conductive members comprise jumpers38, for example, having ends 38.1 which are proportioned to occupy lessthen the full cross-section of the passages but are held therein byengagement of the opposite end of the jumper in another passage.Alternately, a plug (not shown) is provided with a plurality of plugrods slidably fitted into respective passage ends for interconnectingthe ohmic contacts within the passages. These interconnection systemsare useful where the resistor body passages are relatively large andwhere the resistor device is to be used in an application not involvingflow of a fluid through the body passages.

Alternately, the interconnection patterns 24 and 26 are formed on theend surfaces 16 and 18 of the resistor body by brushing or by transferof the pattern from a transfer tape carrier for the interconnectionpattern. For example, in one embodiment of this invention, the resistordevice is processed as indicated at 40, 42, 44, 46 in FIG. 6. The endsof alternate body passages are then counterbored on the body end surface16 as indicated at 16.1 in FIG. 7. Corresponding counterbores 18.1 arethen formed at the opposite ends of the other body passages as indicatedin FIG. 7. Then, transfer tapes 50 and 52 carrying interconnectionpattern materials as indicated at 24a and 26a in FIG. 7 are pressedagainst the opposite ends of the resistor body to transfer the coatingmaterials thereto, the tape carriers 50 and 52 then being withdrawn asindicated by the arrows in FIG. 7 to leave the coatings on the resistorend faces interconnecting appropriate groups of ohmic contacts 22.

It should be understood that although various embodiments of theresistor devices, methods and application systems of this invention havebeen described by way of illustrating the invention, this inventionincludes all modifications and equivalents of the disclosed embodimentsfalling within the scope of the appended claims.

I claim:
 1. A method for making a compact, rugged resistor devicecomprising the steps of providing a body of resistor material ofpositive temperature coefficient of resistivity having outer side andend surfaces and having a plurality of passages extending through thebody in spaced, side-by-side relation to each other between opposite endsurfaces of the body, coating the inner walls of said passages withelectrically conductive material to form ohmic contacts to said resistormaterial, electrically connecting the coatings in alternate ones of saidpassages to form one device terminal, and electrically connecting thecoatings in the other of said passages to form a second device terminal.2. A method as set forth in claim 1 wherein a masking material isapplied to said outer side and end surfaces leaving ends of saidpassages open, said inner passage walls are coated by dipping said bodyinto a coating bath, said body with said coatings formed on said innerpassage walls is heated to an elevated temperature for securing saidcoatings in ohmic contact relation to said resistor material, saidmasking material is removed from said end surfaces to the body, andelectrically conductive interconnection patterns are applied to therespective body end surfaces for electrically connecting said coatingsto form said device terminals.
 3. A method as set forth in claim 1wherein the ends of said alternate passages are counterbored at one ofsaid end surfaces and the ends of said other passages are counterboredat the opposite one of said end surfaces, one of said electricallyconductive interconnection patterns is applied to said one of said endsurfaces for electrically connecting the coatings in said otherpassages, and the other of said electrically conductive interconnectionpatterns is applied to said opposite one of said end surfaces forelectrically connecting the coatings in said alternate passages.
 4. Amethod as set forth in claim 3 wherein said interconnection patterns areapplied to said opposite body end surfaces by transfer of patterns ofelectrically conductive ink from a carrier sheet pressed againstrespective end surfaces of the body.